Abstract: The study of positronium intensity rise in long-chain alkanes shows two kinds of electron traps: the first, which are discharged at low temperature (200 K); the other, which remain populated up to the transition point to the rotator phase; e.g. in C30H62 they are still observed at 328 K. In the rigid phase of even-numbered alkanes o-Ps lifetime is shorter than that of odd-numbered ones, due to the difference in the width of gap between the molecular lamellae. (c) 2006 Elsevier Ltd. All rights reserved.

Abstract: The results of models, extending the Tao-Eldrup model to free volume radii R > 1 nm, are discussed. The influence of free volume size and geometry on ortho-positronium lifetime was examined. Theoretical curves were compared to available experimental data. Then the models were used to obtain information about free volume sizes in MCM-41 porous silica. Numerical analysis of MCM-41 lifetime spectra is difficult due to their considerable complexity. In order to verify the results, various methods of PAL spectra analysis, e.g., deconvolution, fitting, etc., were involved. (c) 2006 Elsevier Ltd. All rights reserved.

Abstract: A review of experimental work on ortho-positronium (o-Ps) lifetime in solids under the pressures up to 1 GPa is presented. Among the effects observed at high pressure one can mention: the disappearance of the energy level for Ps at the reduction of free volume size; pressure induced phase transitions; variation of Ps formation intensity with time; increase of o-Ps lifetime after intercalation of high pressure gas to the paraffin samples. (c) 2006 Elsevier Ltd. All rights reserved.

Abstract: The quadrupole interaction at Cd-111 impurity nuclei in the intermediate-valence compound YbAl2 has been measured under pressure up to 80 kbar by the time-differential perturbed angular correlation spectroscopy. It was found that the quadrupole frequency nu(Q) measured on the Cd-111 nuclei located at the Al sites in YbAl2 varies nonlinearly and increases by almost four times with the pressure increase up to 80 kbar. A linear correlation between the mean Yb valence, derived from Yb L-3 OFY-XAS and RXES measurements, and the electric field gradient (the quadrupole frequency nu(Q)=eQV(zz)/h) has been observed. The quadrupole frequencies on Cd-111 in the GdAl2, YbAl3, TmAl3, and CaAl2 compounds have been also measured. The possibility of determining the valence of Yb in the Yb compounds with p metals from the relation nu(Q)(upsilon(P))=nu(2)+(nu(3)-nu(2))upsilon(P) has been considered.

Abstract: The results of the surface treatment of commercial aluminum by nitrogen ion implantation at 120 keV and implanted fluences ranging from 3 X 10(17) to 1.1 x 10(18) ions/cm(2) are reported. The treatment was found to lead to the formation of the hexagonal phase AIN, a decrease in strain and an increase of crystallite sizes. The modification of the surface layer so produced was thought to be a cause of a measured increase in surface microhardness and corresponding decrease in friction coefficient and wear measured in pure methanol. Oxygen found in the surface layers was also thought to play a significant role in determining tribological performance. (c) 2007 Published by Elsevier Ltd.

Abstract: 4-Chloro-2-methoxybenzoates of light lanthanides(III) were obtained as mono-, di- or trihydrates with metal to ligand ratio of 1:3 and general formula Ln(C8H6ClO3)(3)center dot nH(2)O, where n=1 forLn=Ce, Pr, n=2 for Ln=Nd, Sm, Eu, Gd and n=3 forLn=La. The complexes were characterized by elemental analysis, IR spectra, then nogravimetri c studies, X-ray diffraction and magnetic measurements. The carboxylate group appears to be a symmetrical bidentate, chelating ligand. All complexes seem polycrystalline compounds. Their thermal stabilities were determined in air. When heated they dehydrate to form anhydrous salts which next are decomposed to the oxides of the respective metals. The solubilities of light lanthanide(III) 4-chloro-2-methoxybenzoates in water at 293 K are of the order of 10(-5) mol dm(-3). The magnetic moments were determined over the range of 77-300 K. They obey the Curie-Weiss law. The values of mu(eff) calculated for all compounds are close to those obtained for Ln(3+) by Hund and Van Vleck. The results indicate that there is no influence of the ligand field of 4f electrons on lanthanide ions and the metal ligand bonding is mainly electrostatic in nature.

Abstract: The physico-chemical properties and thermal stability in air of Co(II) 2,3-, 3,5- and 2,6-dimethoxybenzoates were compared and the influence of the position of -OCH3 substituent on their thermal stability was investigated. The complexes are crystalline, hydrated and anhydrous salts with pink colour. The carboxylate ions are a monodentate, bidentate chelating or bridging and tridentate groups. The thermal stability of analysed Co(II) dimethoxybenzoates was studied in the temperature range of 293 - 1173 K. The positions of methoxy groups in benzene ring influence the thermal properties of studied complexes. Their different thermal properties are markedly connected with the various influences of inductive, mesomeric and steric effects of the OCH3 substituent on the electron density in benzene ring. The magnetic susceptibilities of the complexes were measured over the range of 76 - 300 K and the magnetic moments were calculated. The results show that 2,3-, 3,5- and 2,6-dimethoxybenzoates of Co(II) are high-spin complexes with weak ligand fields.

Abstract: Positron lifetime spectra were measured for a series of odd- numbered n-alkanes from C11H24 to C19H40 as a function of temperature and pressure. The ortho-Ps lifetimes in the rotator phase and the increase in intensity at the transition to that phase can be explained by location of Ps in the vicinity of kink-type conformers. The relation between o-Ps lifetime and molecule length can be described in the framework of extended Tao-Eldrup model. One can eliminate the effect of intensity rise in time by sample illumination or by application of high pressure. A decrease in temperature by 1 K is equivalent to an increase in pressure by about 4 MPa.

Abstract: Temperature dependence of ortho-positronium lifetime in some porous materials was investigated. Above the room temperature the extended Tao-Eldrup model works rather well, however at low temperatures one observes in all samples the lifetimes evidently longer than expected. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Positron annihilation lifetime spectroscopy was used to observe the effects of argon intercalation in some solid long-chain alkanes at high pressure. The ortho-Ps lifetime rises with argon pressure, which means increase of free volumes in the alkane structure. The range of pressures in which the rotator phase exists increases, comparing. to pure alkane. In n- heptadecane, n-nonadecane, and possibly n-heneicosane, a stepwise change of ortho-Ps lifetime and intensity at 12 MPa is observed, suggesting the transition to a new kind of the rotator phase. The transition rate is low, final lifetime value is approximate to 3.3 us. Despite a large size of free volumes corresponding to such a lifetime, their compressibility is found negligible up to the pressure of 90 MPa. At low pressures the compressibility of free volumes in the rotator phase is negative. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: The positron annihilation lifetime spectroscopy was applied to study the porous Vycor glass annealed at the temperatures between 950 and 1050 K. With increased temperature the spectra show significant decrease in o-Ps lifetime and intensity in the pores, while the rise of intensity in the small intermolecular voids is observed. Similar changes are observed as a function of annealing time at fixed temperature depending on the time and temperature of thermal treatment. They can originate from partial crystallization of the silica in the material under study.

Abstract: Positronium formation probability was investigated as a function of irradiation time and temperature for some long- chain saturated hydrocarbons (alkanes). Intensity of ortho- positronium component in the lifetime spectrum depends on the concentration of trapped excess electrons, but also on concentration of radicals and ions acting as electron scavengers. The results indicate the existence of two kinds of electron traps; those which are not emptied by thermal treatment disappear after several days of positron irradiation.

Abstract: Some physico-chemical properties of the 2-chloro-5- nitrobenzoates of Co(II), Ni(II) and Cu(II) were studied. The complexes were obtained as hydrates with a metal ion to ligand ratio of 1:2. All the complexes were polycrystalline compounds. Their colours depended oil the nature of the central ion: pink for the Co(II) complex, green for the Ni(II) and blue for the Cu(II) compounds. Their thermal decomposition was studied in the range of 293-523 K because it was found that the 2-chloro- 5-nitrobenzoates decomposed explosively oil heating in air above 523 K. The hydrated complexes lost crystallization water molecules in one step. From the results, it appeared that no transformation of the nitro group to nitrite occurred during the dehydration process. Their solubilities in water at 293 K were of the order of 10(-2) mol dm(-3) for the Ni(II) and Co(II) complexes and 10(-4) mol dm(-3) for that of Cu(II). The magnetic moment values of Co2+, Ni2+. and Cu2+ in the 2-chloro- 5-nitrobenzoates, experimentally determined in the range, of 76-303 K ranged from 3.48 to 3.82 mu(B) for Co(II), from 2.69 to 3.08 mu(B) for the Ni(II) and from 1.87 to 2.00 mu(B) for the Cu(II) complex. The 2-chloro-5-nitrobenzoates of Co(II), Ni(II) and Cu(II) followed the Curie-Weiss law.

Abstract: Physico-chemical properties of 4-chloro-2-nitrobenzoates of Co(II), Ni(II), and Cu(II) were studied. The complexes were obtained as mono-and trihydrates with a metal ion to ligand ratio of 1:2. All analysed 4-chloro-2-nitrobenzoates are polycrystalline compounds with colours depending on the central ions: pink for Co(II), green for Ni(II), and blue for Cu(II) complexes. Their thermal decomposition was studied only in the range of 293-523 K, because it was found that on heating in air above 523 K 4-chloro-2-nitrobenzoates decompose explosively. Hydrated complexes lose crystallization water molecules in one step and anhydrous compounds are formed. The final products of their decomposition are the oxides of the respective transition metals. From the results it appears that during dehydration process no transformation of nitro group to nitrite takes place. The solubilities of analysed complexes in water at 293 K are of the order of 10(-4) -10(-2) mol dm(-3). The magnetic moment values of Co2+, Ni2+ and Cu2+ ions in 4-chloro-2- nitrobenzoates experimentally determined at 76-303 K change from 3.89 to 4.82 mu(B) for Co(II) complex, from 2.25 to 2.98 mu(B) for Ni(II) 4-chloro-2-nitrobenzoate, and from 0.27 to 1.44 mu(B) for Cu(II) complex. 4-chloro-2-nitrobenzoates of Co(II), and Ni(II) follow the Curie-Weiss law. Complex of Cu(II) forms dimer.

Abstract: Some physicochemical properties of 2-chloro-4-nitrobenzoates of Co(II), Ni(II), and Cu(II) were studied. The complexes were obtained as mono- and dihydrates with a metal ion-ligand mole ratio of 1 : 2. All complexes are polycrystalline compounds. Their colours depend on the kind of central ion: pink for Co(II) complex, green for Ni(II), and blue for Cu(II) complexes. Their thermal decomposition was studied only in the range of 293 K-523 K because it was found that on heating in air above 523 K 2-chloro-4-nitrobenzoates decompose explosively. Hydrated complexes lose crystallization water molecules in one step. During dehydration process no transformation of the nitro group to nitrito one took place. Their solubilities in water at 293 K are of the orders of 10(- 3)-10(-2) mol dm(-3). The magnetic moment values of 2-chloro-4- nitrobenzoates determined in the range of 76 K-303 K change from 3.48 mu(B) to 3.82 mu(B) for Co(II) complex, from 2.24 mu(B) to 2.83 mu(B) for Ni(II) 2-chloro-4-nitrobenzoate, and from 0.31 mu(B) to 1.41 mu(B) for Cu(II) complex. 2-Chloro-4- nitrobenzoates of Co(II) and Ni(II) follow the Curie-Weiss law, but the complex of Cu(II) forms dimer.

Abstract: Four new complexes of 2,3,4-trimethoxybenzoic acid anion with manganese(II), cobalt(II), nickel(II) and copper(II) cations were synthesized, analysed and characterized by standard chemical and physical methods. 2,3,4-Trimethoxybenzoates of Mn(II), Co(II), Ni(II) and Cu(II) are polycrystalline compounds with colours typical for M(II) ions. The carboxylate group in the anhydrous complexes of Mn(II), Co(II) and Ni(II) is monodentate and in that of Cu(II) monohydrate is bidentate bridging one. The anhydrous complexes of Mn(II), Co(II) and Ni(II) heated in air to 1273 K are stable up to 505-517 K. Next in the range of 505-1205 K they decompose to the following oxides: Mn3O4, CoO, NiO. The complex of Cu(II) is stable up to 390 K, and next in the range of 390-443 K it loses one molecule of water. The final product of its decomposition is CuO. The solubility in water at 293 K is of the order of 10(-3) mol dm(- 3) for the Mn(II) complex and 10(-4) mol dm(-3) for Co(II), Ni(II) and Cu(II) complexes. The magnetic moment values of Mn2+, Co2+, Ni2+ and Cu2+ ions in 2,3,4-trimethoxybenzoates experimentally determined in the range of 77-300 K change from 5.64-6.57 mu B (for Mn2+), 4.73-5.17 mu B (for Co2+), 3.26-3.35 mu B (for Ni2+) and 0.27-1.42 mu B (for Cu2+). 2,3,4- Trimethoxybenzoates of Mn(II), Co(II) and Ni(II) follow the Curie-Weiss law, whereas that of Cu(II) forms a dimer.

Abstract: Positron annihilation lifetime spectroscopy as a technique of porous materials investigation is still under development. Both measurement conditions and lifetime spectra analysis methods have to be adapted to the requirements of positron porosimetry. If complex positron annihilation lifetime spectra of porous materials are processed, numerical methods of lifetime spectra analysis have to be used with a particular care. Among various methods of positron annihilation lifetime spectra analysis available, the most popular ones were tested implemented in programs MELT and LT. It was found, from the simulated spectra study, that a large number of counts in a spectrum is needed to obtain reliable results of analysis. Even then only an approximate solution is available. Determination of the approximation nature helps in a further interpretation of the results and in positron annihilation lifetime spectrometer setting adequate to the problem. For this purpose, a few complex spectra obtained from positron beam facility were tested. The results show a negligible influence of resolution function on long lifetime components, while the precise estimation of the background level is crucial. Also some distortions caused by imperfection of numerical analysis methods are pointed out.

Abstract: A series of C45 steel samples was burnished by shot peening with varying time of treatment. The samples were investigated by nondestructive positron annihilation techniques: angular correlations of two-quantum annihilation radiation and positron annihilation lifetime spectroscopy. To determine residual stresses present under burnished surface the same samples were studied by destructive Davidenkov method. Change of absolute value of the weighted average of residual stresses over positron range in the series of the samples is in good agreement with change of S/W ratio obtained by angular correlations of two-quantum annihilation radiation. Both parameters increase during first 10 minutes of shot peening and then stabilize. Positron annihilation lifetime spectra allowed to identify two types of structural defects: smaller ones like vacancies or dislocations and bigger - probably clusters of vacancies. Increase in shot peening time causes reduction of positron trapping rate and lifetime rise in bigger defects.

Abstract: X-ray diffraction, Mossbauer spectroscopy and magnetization measurements were used to study the structure and some magnetic properties of Fe50Ge50 and Fe62Ge38 prepared by mechanical alloying from the elemental powders. In both cases in the early stages of milling the intermediate paramagnetic FeGe2 phase was formed. The mechanical alloying process of Fe50Ge50 resulted in the formation of the paramagnetic FeGe (B20) phase with an average crystallite size of about 15 nm. In the case of the Fe62Ge38, the ferromagnetic Fe5Ge3 (beta) phase with a Curie temperature of about 430 K was obtained. The average crystallite size was about 9 nm. The average hyperfine magnetic field of about 16 T allowed it to determine that more than four germanium atoms exist in the nearest environment of the Fe-57 isotopes in the Fe5Ge3 phase. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: A short review of experiments concerning the influence of high pressure on positron annihilation in molecular solids is given. To this kind of experiments belong e.g. pressure inhibition of positronium formation, pressure induced phase transitions, and free volume swelling in the media, intercalated by high pressure gas.

Abstract: This paper represents a fragment of systematic studies on positron annihilation in solid saturated hydrocarbons (alkanes, paraffins) from C17H36 to C30H62. The Tao-Eldrup model and its extensions allow to identify the places in the crystalline structure where positronium is trapped, and estimate respective trap sizes. In the rigid phase I positronium locates mainly in two-dimensional empty spaces between the lamellae of crystal structure. Additional free volumes exist in high temperature phases due to appearance of n-alkane conformers representing a non-planar molecule distortion. The rise of ortho-positronium intensity with the sample irradiation dose, which was seen earlier in polymers, is observed also in n-alkanes. One can find similarities between the ortho-positronium annihilation in crystalline alkanes and polymers, like exponential character of that rise, and the a trap bleaching by illumination. It confirms the same origin of the intensity growth, i.e. the accumulation of trapped electrons. Two kinds of electron traps were found. The time constant of electron accumulation in the traps depends on the carbon chain length and varies with temperature in different way for even- and odd-numbered alkanes.

Abstract: Positron lifetime and pressure-volume-temperature experiments were performed as function of T (100-485 K) and P (10(-5) Pa- 500 MPa) on a fluoroelastomer. From positron annihilation lifetime spectroscopy, analyzed with LT9.0 assuming a dispersion in the e(+) (tau(2)) and o-Ps (tau(3)) lifetime, the mean, < v(h)>, and the width (standard deviation), sigma(h1) of the hole size distribution were calculated. The pressure- volume-temperature data were analyzed using the Simha-Somcynsky hole-lattice theory to estimate the specific hole free volume V-f. From a comparison of V-f with (v(h)) the specific hole number, N-h(1), is estimated. The data from thermal expansion and pressure experiments show complete agreement when taking into account the compressibility of the occupied volume.

Abstract: Positron annihilation lifetime spectroscopy was applied to study modification of sub-nanometer voids in one of n-alkanes subjected to high pressure. Without a contact with gaseous atmosphere the sizes of voids behaved as expected - decreased with the rise of pressure. When the pressure was applied with argon penetrating the samples - the voids expanded. After the phase transition the structure damage allowed positronium, trapped in voids, to escape out of a damaged sample. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The microstructure of the free volume and its high-pressure dependence in fluoroelastomeric copolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well as vinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF/HFP22, was studied by pressure- volume-temperature experiments (PVT, P = 0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, P = 0.1-448 MPa, all at 22.5 degreesC). Employing the Simha-Somcynsky equation of state the excess (hole) free volume fraction h and the specific free and occupied volumes, V-f = hV and V-occ = (1- h)V, were estimated from the specific total volume V. The pressure variation of these volumes and their compressibilities are discussed. We found that the occupied volume V-occ exhibits a remarkable compressibilitY7 K-occ approximate to 2.4 x 10(-4) MPa-1, while it shows (for T > T-g) almost no thermal expansion. The PALS spectra were analyzed using the new routine LT9.0 assuming dispersion in both the positron (tau(2)) and orthopositronium (o-Ps) lifetimes (tau(3)). We speculate that positrons (e(+)) may show Anderson localization at sites of the entire empty volume possibly assisted by an affinity to fluorine atoms. From the o-Ps lifetime parameters the (excess) free volume hole size distribution, its mean value , and dispersion sigma(h) were calculated. From a comparison of with V-f the specific hole number N-h' was estimated. N-h' was determined to be constant and the same in compression and thermal expansion experiments. When taking correctly into account the compressibility of the occupied volume V-occ the discrepancy in V vs plots between compression and thermal expansion experiments discussed in the literature disappears.

Abstract: The porous structure of glasses incorporated by ruthenium tris bipyridyl chloride (further Rubpy) and adsorbed by Rubpy is studied by positron annihilation lifetime spectroscopy and liquid nitrogen adsorption method. The first method provides the radius of the pores in the range of 0.6-4 nm and their distribution. The second method provides the radius of the pores in the range of 1-150 nm, their total volumes, size distribution and specific surface area. In the first method both closed and opened pores can be seen when the measurements are made on bulk samples as in this work. When thin films are measured, one has to use a capping material to prevent backscattering. The dimension of the pores can be controlled by the catalyst used for sol-gel formation. When Rubpy is prepared by adding its salt to precursor solution catalyzed by HCl, the pore size increases because of stress exhibited by large molecules. The pore size decreases with adding of Rubpy in precursor solution, catalyzed by HF and HNO3. Glass doped by Rubpy is less sensitive to the type of catalyst, than undoped glasses. A comparative study was performed on undoped glasses prepared by the identical method. We shall discuss the similarities and differences of the two methods.

Abstract: Three stages of thermal and chemical processing of Vycor glass were carried out. Raw material, the glass with small and big pores was studied. In each stage of study various crystallinity degrees of investigated material were obtained. The relation between crystallinity degree and the intensity of o-Ps components was determined. In nonporous material a high correlation between o-Ps intensity and crystallinity degree was found. In porous material the decrease in total o-Ps intensity can be a measure of the crystallinity degree.

Abstract: Three methods of determination of 3 gamma o-Ps decay intensity are compared. The estimate of 3 gamma fraction obtained from classical lifetime measurements can be distorted due to a higher absorption of the gamma quanta from the continuous spectrum of three-quantum decay, compared to two-quantum one.

Abstract: The temperature dependence of o-Ps lifetime in the pores calculated from extended Tao-Eldrup model was compared to the experimental data, collected for a set of silica gels synthesized using the polymer template technique. For the sample with average pore radii 2.0 nm rather good agreement between the model and experiment was found. For the sample with narrow pores (near 1.0 nm) the lifetime spectrum was complex. Except the range of highest temperatures the lifetimes are far from model calculations.

Abstract: The physicochemical properties and thermal stability in air of 2,3,4- and 3,4,5-trimethoxybenzoates of Mn(H), Co(H), Ni(H), and Cu(II) were compared and the influence of the position of - OCH3 substituent on their thermal stability and magnetic properties was investigated. The complexes of these series are crystalline, hydrated or anhydrous salts with colours typical of M2+ ions. The carboxylate ion is a monodentate and bidentate chelating or bridging group. The thermal stability of complexes was studied in the temperature range of 293-1273 K. The solubility in water at 293 K for these analyzed complexes is in the order of 10(-3)-10(-4) mol dm(-3) for 2,3,4- trimethoxybenzoates and 10(-2)-10(-4) mol dm(-3) for 3,4,5- trimethoxybenzoates. The magnetic susceptibilities for trimethoxybenzoates were measured over the range of 76-303 K and their magnetic moments were calculated. The results show that compounds of Mn(H), Co(H), and Ni(II) are high-spin complexes but those of Cu(II) form dimers.

Abstract: The complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with 3,4,5-trimethoxybenzoic acid anion of the formula: M(C10H11O5)(2)(.)nH(2)O, where n = 6 for Ni(II), n = 1 for Mn(II), Co(II), Cu(II), and n = 0 for Zn, have been synthesized and characterized by elemental analysis, IR spectroscopy, X-ray diffraction measurements, thermogravimetry and magnetic studies. They are crystalline compounds characterized by various symmetry. They decompose in various ways when heated in air to 1273 K. At first, they dehydrate in one step and form anhydrous salts. The final products of decomposition are oxides of the respective metals (Mn2O3, Co3O4, NiO, CuO, ZnO). The solubilities of the analysed complexes in water at 293 K are in the orders of 10(-2) - 10(-4) mol dm(-3). The magnetic susceptibilities of the Mn(II), Co(II), Ni(II) and Cu(II) complexes were measured over the range of 76-303 K and the magnetic moments were calculated. The results show that the 3,4,5-trimethoxybenzoates of Mn(II), Co(II) and Ni(II) are high-spin complexes but that of Cu(II) forms a dimer [Cu- 2(C10H11O5)(4)(H2O)(2)]. The carboxylate groups bind as monodentate or bidentate chelating or bridging ligands.

Abstract: The 2,3- and 3,5-dimethoxybenzoates of Co(II), Ni(II) and Cu(II) were synthesized as solids and their magnetic, spectral and thermal properties studied. The complexes are hydrated or anhydrous compounds which possess colours typical of the M(II) ions. Their thermal stabilities were examined in air and nitrogen and the gaseous and solid state decomposition products were also identified. The magnetic susceptibilities of the complexes were measured over the temperature range 4.4-300 K and the magnetic moments were calculated. The results show that the 2,3- and 3,5-diniethoxybenzoates of Co(II) and Ni(II) are high-spin complexes with weak ligand fields, and that the complexes of Cu(II) form dimers.

Abstract: The present study reports on improvements of the tribomechanical properties of commercial aluminum especially wear and hardness by nitrogen implantation. The implantation process has been carried out at doses 5 x 10(16), 1 x 10(17) and 6 x 10(17) ions/cm(2) and energy 180 keV. The tribological tests for the friction coefficients and wear were made on a pin-on-disk tribotester with the load 0.98 N. XRD analyses were performed to obtain a characterization of the surface of the implanted sample. The implanted specimens were also annealed at 400 degrees C in addition to the non-implanted specimen for referring. The hardness showed remarkably significant improvement at the higher dose, 6 x 10(17) ions/cm(2). Before annealing, the friction coefficients of the implanted samples were nearly unchanged. After annealing at 400 degrees C, these coefficients exhibited a relative decrease from about 0.33-0.22 at dose 1 x 10(17) ions/cm(2). On the other hand, the wear rates of the implanted samples and the countersample exhibited a great decrease after annealing the samples of aluminum at 400 degrees C whereas the wear decreased from about 38 to 3 mu m(2) for the samples at the higher dose, 6 x 10(17) ions/cm(2). (c) 2005 Elsevier Ltd. All rights reserved.

Abstract: Template transformation in MCM-41 material during thermal treatment under different conditions was investigated on the basis of thermogravimetry (TG-DTA), X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS). Micelle templated silica was prepared using C18 trimethylammonium bromide. The pore structure of MCM-41 samples obtained after removal of the surfactant in air, argon flow and vacuum was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K and XRD experiments. The TG-DTA experiments confirm the mechanism of the template removal known from literature. However, the sequence of the processes during thermal treatment of as-synthesized sample and temperature of transformations depended strongly on the presence of oxygen and the heating rate. The main template degradation took place below 573 K and was independent of the kind of atmosphere above the sample. Residual carbonaceous species are removed from pores and the external surface of MCM-41 silica upon heating to 823 K by combustion or evaporation. The latter process as well as translocation of liquid-like products of template degradation from the pore interior to external surface was confirmed by PALS experiment in vacuum.

Abstract: Room temperature Fe-57 Mossbauer spectroscopy and X-ray diffraction measurements are reported for a series of Y1- xTixFe2 polycrystalline samples. Our research shows that except for x = 0 and x = 1, the samples are a mixture of two C15 and C14 pseudobinary Laves phases. Some Ti atoms occupy the crystallographic positions 8a characteristic of the Y atoms in the cubic C15 structure while the rest of them form the hexagonal C14 phase. However, the lattice constant dependencies on concentration x suggest the occurrence of Y atoms in the positions 4f characteristic of Ti atoms in the hexagonal C14 structure. The partial substitution of Ti atoms for Y atoms is responsible for the HMF changes in the cubic phase. Contrary to the situation taking place in the Zr1-xTixFe2 compounds, magnetic arrangement in the pseudobinary hexagonal phases is not observed at room temperature.

Abstract: Mechanical alloying method was used to prepare nanocrystalline Co52Fe26Ni22 alloy. X-ray diffraction was applied for determination of the structure of the alloy. During milling Co- based solid solution with f.c.c. lattice was formed. After 80 h of synthesis the lattice parameter was equal to 0.3575 nm while the average grain sizes and the mean level of internal strains were about 24 nm and 0.72%, respectively. Mossbauer spectroscopy was adopted to characterize the local atomic order of the Co52Fe26Ni22 alloy. In the nearest neighbourhood of Fe- 57 isotopes there are at least six Co atoms, three Ni atoms and three Fe atoms giving the hyperfine magnetic field equal to 32.45(1) T. Magnetization measurements allowed to determine the effective magnetic moment of the Co52Fe26Ni22 alloy to be equal to 1.63 mu(B) per formula unit. Curie temperature of the obtained alloy is equal to 1000 K. (C) 2004 Kluwer Academic Publishers.

Abstract: Positron annihilation lifetime spectra in solid n-pentacosane were measured as a function of the temperature and positron irradiation time. Phase IV was found to be well visible in the cooling runs due to its easy supercooling; a small increase of ortho-Ps lifetime in phase IV compared to low-temperature phase indicates that predominant intramolecular defects are end- gauche ones. The effect of excess electron accumulation, typical mainly of polymers, was also observed; two kinds of electron traps were found. The main trap component had activation enthalpy 0.6 eV and disappeared at about 250 K; an additional fast accumulating component existed up to the temperatures near the transition to the rotator phase. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The lifetime of ortho-positronium (o-Ps) in solid phases of long-chain n-alkanes was measured. In even-numbered alkanes, only one phase transition is observed; a very long lifetime is related to large free volumes near kink-shaped conformers. In odd-numbered alkanes (n less than or equal to 25), another solid phase (phase IV) is characterized by slightly increased o-Ps lifetime, which means the domination of end-gauche and double-gauche intramolecular defects. In C29H60 the structure of defects changes continuously in the whole range of phase IV. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The lifetime Of ortho-positronium in solid naphthalene shortens with the rise of pressure as a result of diminishing the free spaces in which it is trapped. At the pressure approximate to100 MPa the size of these spaces is too small to accommodate the Ps atom (no energy level in the well), and the intensity of ortho-Ps component. initially weakly dependent on pressure, abruptly drops to zero. The narrow range in which ortho-Ps disappears indicates uniformity of Ps-accommodating voids. The nonmonotonous dependence of free positron lifetime is discussed in the framework of the blob model. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The increase of positronium, formation probability with the time of exposure to positron radiation, observed until now mainly in polymers, is shown for the case of polycrystalline n- nonadecane. Sample illumination by visible light restores the initial low value of positronium intensity owing to elimination of trapped electrons. Slowing down the growth of positronium intensity with prolonged positron irradiation is ascribed to inhibiting action of radicals. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Positron lifetime spectra in solid long-chain alkanes were measured as a function of the storage time with positron source between the samples, and as a function of temperature and pressure, Large differences of the time constant of I-3 increase were observed for odd-numbered and even-numbered molecules. High pressures also induced the I-3 instability, but in this case the I-3 intensity decreased. In the rigid solid phase the o-Ps lifetime in even-numbered alkanes was shorter than in odd ones, which was due to different thickness of the empty layer between the lamellas in the crystal structure.

Abstract: Porous glasses prepared by the sol-gel technique have a variety of applications when incorporated by photonic materials: tunable lasers, sensors, luminescence solar concentrators, semiconductor quantum dots, biological markers. The known methods of pore size determinations, the nitrogen adsorption and mercury porosimetry allow to determine the sizes of open pores. Positron annihilation lifetime spectroscopy (PALS) allows to determine pore sizes also of closed pores. As an example we have performed measurements of non-doped zirconia- silica-polyurethane (ZSUR) ormocer glasses and the same glasses doped with lead sulfide quantum dots. The pore radii range between 0.25-0.38 nm, total surface area 15.5-23.8 m(2)/g. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The removal of alkyltrimethylammonium template from MCM-41 ordered silica was investigated, using positron annihilation lifetime spectroscopy (PALS). The lifetime spectra of positrons were measured as a function of temperature for several samples with different template alkyl chain length (different pore radius). At low temperatures the template interior behaves like a liquid, and the "bubble model" of positronium in liquid can be applied. The change of template structure starts at 380-400 K, and at 520 K the process of template removal is completed. Analysis of o-Ps lifetime components suggests the following stages: breaking long micelle cylinders, rise of the gaps between the fragments, and pore evacuation.

Abstract: Measurements of X-ray diffraction and Mossbauer effect were made on a series of Y1-xScxFe2 polycrystalline samples (x = 0.0, 0.2, 0.5, 0.7, 0.8, 1.0). It was found that the system has the cubic MgCu2 structure except for x = 1.0 where hexagonal MgZn2 structure type is stable. The lattice constant decreases with increasing x. The results of Mossbauer effect study at room temperature show that the easy axes of magnetization remain in the <111> direction for the cubic samples and <100> for the hexagonal one. Curie temperatures T-C were determined from temperature dependence of the hyperfine magnetic fields. The concentration dependence of the hyperfine fields and T-C show similar trends, exhibiting a maximum at x = 0.8. The magnetic and structural behaviour show that, in spite of being isoelectronic, the substitution of Y by Sc induces clear changes in the structural and magnetic properties of the compounds under investigation.

Abstract: Positron lifetime spectra in solid n-alkanes C19H40, C20H42 and C21H44 were measured. The effect of irradiation by positron source (increase of ortho-positronium intensity I-3), known from polymer studies, was found to appear also in alkanes; at about 100 K it can lead to doubling the intensity. The time constant of I-3 rise in n-eicosane increased with temperature, reaching the value of 60 h, while it decreased (down to 1 h) in its odd-numbered neighbours. The difference in ortho-Ps lifetimes between odd- and even-numbered alkanes can be explained by different thickness of interlamellar layer in the crystal structure. The variation of I-3 after application of high pressure was also observed, but in this case the intensity decreased with time. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The usefulness of positron annihilation techniques to the study of porous med a seems to be well proven. The models of pick-off annihilation for cylindrical and rectangular geometries give identical tau vs. pore size dependence, except the range of transverse size below 3 nm. Approximations contained in the model and the influence of Ps interaction with the medium (other than pick-off) on the observed o-Ps lifetime are discussed. (C) 2003 Elsevier Ltd. All rights reserved.

Abstract: The effect of high pressure on positron lifetime spectra in resorcinol was investigated. The o-Ps lifetime decreases with pressure continuously reaching the value which needs redefinition of DeltaR value used in the Tao-Eldrup model. The intensity of long-lived component unexpectedly rises with pressure increase, but it 450 MPa a drop of that intensity can indicate the approach of the minimal size of free volume able to trap Ps. preliminary data for hydroquinone are also presented. (C) 2003 Elsevier Ltd. All rights reserved.

Abstract: The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380-400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m(2)/g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho-positronium, atoms leave the pores before annihilation. (C) 2003 Elsevier Inc. All rights reserved.

Abstract: Two Vycor glasses were investigated using PALS method. The model of pick-off annihilation in cylindrical free volume was applied to evaluate the pore radii. The o-Ps mean lifetime value in the pores changed from 6.5 to 40 ns in one glass, while from 13 to 51 ns in the other (softer) one. The determined pore radii, using a cylindrical shape model, were from 0.5 to 1.4 nm and from 0.7 to 1.6 nm, respectively.

Abstract: Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results. (C) 2003 Elsevier Science (USA). All rights reserved.

Abstract: The properties of positronium annihilating in the micellar template in MCM-41 materials were investigated. For alkyltrimethylammonium bromide surfactants (C-n = 14,16,18) the template interior is liquid-like; the bubble model of positronium, elaborated for bulk liquids, can be applied. The equivalent surface tension is linearly dependent on temperature, like in classic liquids. A very long-lived component in o-Ps lifetime spectrum (approximate to 20 ns) can be ascribed to o-Ps in cracks in the elongated template structure. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: The lifetime of para-positronium in raw materials of MCM-41 class was measured as a function of temperature. It was found that the lifetime shortens when the template filling is removed and empty pores are created. Rough data are given also for phase transition in n-nonadecane. The electron contact density eta on the positron bound into positronium atom in the condensed media, determined in experiments, seems to be related more to the size of positronium trap than to the electronic properties of the medium. The role of contact density eta in the estimation of ortho-positronium lifetime in the bulk is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: Positron lifetime spectra in ordered mesoporous silica were measured. In the raw material (still containing the template) the voids in micellae reaching 1 nm in size were observed. Ortho-positronium (o-Ps) lifetime was found to be very sensitive to the carbon deposits left in pores after pyrolysis. In highly dispersed materials the relation between the pore diameter and positronium lifetime is shadowed by o-Ps escape from the pores to extragranular spaces. A very slow thermalization of o-Ps and increased lifetime of free positrons in low-density bulk were also observed.

Abstract: Pore formation in Vycor glass subjected to annealing at 530degreesC and 550degreesC for different periods of time ranging from 4 to 48 h was studied. The o-Ps lifetime in such a medium was in the range 60-100 ns, while the intensity of this component varied from 20% to 28%. The values of pore radii extracted from PALS data agreed well with those from other porosimetric methods if DeltaR applied in cylindrical model calculations was about 0.18 nm. (C) 2003 Elsevier Science Ltd. All rights reserved.

Abstract: This study concerns the crystallization process observed in three-component Vycor glass heated in the temperature range from 875 to 975 K. PALS and X-ray diffraction results confirmed partial crystallization of silica in the studied material depending on the time and temperature of thermal treatment. It is shown that PALS can be treated as a new method for investigation of the crystallinity degree. Increased crystallization degree obtained from X-ray diffraction data is perfectly correlated with o-Ps intensity decrease determined from PALS spectra for each temperature.

Abstract: The magnetic structure and hyperfine interactions in the polycrystalline pseudobinary system (Zr1-xTix)Fe-2 for x less than or equal to 0.2 and x greater than or equal to 0.7 were studied using the techniques of neutron diffraction and the Mossbauer effect. The results of our measurements reveal the cubic C 15 and the hexagonal C 14 crystal structure for low and high Ti atom concentration x, respectively. For the ferrimagnetic region of the C15 structure the reduction from 1.58(5) to 1.30(6) muB of Fe magnetic moments with increasing x at room temperature was found. The presence of two hyperfine magnetic fields in the antiferromagnetic C14 phases suggests the existence of small magnetic moments on Fe atoms at 2a in addition to those present at the 6h sites. The magnetic structure refinement for the (Zr0.3Ti0.7)Fe-2 alloy yielded magnetic moments perpendicular to the c axis of 0.19(10) mu(B) at Fe 2a sites at 10 K. The results confirm the spin-canted antiferromagnetic structure with a canting angle of 8degrees at 10 K in the pseudo-binary phase.

Abstract: Iron layers with thickness of 70-200 nm were prepared by electrodeposition on the polycrystalline Cu substrate at different parameters of the electrolysis. The surface morphology of the iron layers and their local magnetic properties were studied using scanning tunneling microscopy (STM) and conversion electron Mossbauer spectroscopy (CEMS). STM studies revealed the granular structure of the surface of the electrodeposited iron layers with the roughness depending on the temperature of the bath, pH value and the current density. The influence of these electrodeposition parameters on the thickness of the iron layers became manifest in their magnetic anisotropy, as seen by CEMS. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: X-ray diffraction and Mossbauer spectroscopy were used to study the structure and hyperfine interactions in Fe80Mo20 and Fe50M50 prepared by mechanical alloying. Two solid solutions, i.e. Fe(Mo) and Mo(Fe) with b.c.c. lattice were formed during milling of Fe80Mo20. Mossbauer spectra revealed different magnetic arrangements in these solid solutions. In the case of Fe50Mo50 no amorphisation was observed, as literature data suggest. During mechanosynthesis of Fe50Mo50 a paramagnetic Mo(Fe) solid solution was probably formed. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The author is discussing the parameters appearing in the Tao- Eldrup (TE) model describing the ortho-positronium (o-Ps) lifetime dependence on the size of free volume in which Ps is trapped. Parameter values are not universal, applicable to all media. The Ps penetration to the bulk should depend on Ps work function; the o-Ps decay rate is strongly influenced by the contact density factor. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The lifetime spectra of positrons annihilating in vapour- absorbing polymers were measured in order to gain information about the change of dimensions and number of free volume holes during absorption and desorption. Absorption of water, methanol and ethanol vapours in melamine-formaldehyde resin leads to the shortening of ortho-positronium lifetime that can be explained by predominant filling larger holes by absorbed molecules, leaving smaller ones empty and accessible for ortho- positronium. Removal of molecules by outpumping the vapour increases the lifetime. It was found that the intensity of positronium formation does not follow the rule of proportionality to the number of free volume sites. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The sources of possible inconsistency between the models of trapped positronium annihilation and experimental data are discussed. There are two such sources: excessive simplifications in the models of pick-off process and appearance of other processes not accounted in the models. In the case of the smallest traps, the penetration depth of ortho- positronium (o-Ps) wavefunction into the bulk can be larger than usually assumed. In traps of sizes exceeding I run the population of excited levels in the well has to be taken into account. For such large voids the sources of divergence of the experiment and model predictions can be: escape of o-Ps from open pores to outside, chemical interaction of o-Ps with pore surfaces (adsorbate layers), the presence of impurities inside free volumes. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The notion of groups of motions as groups of elementary changes in quantum states leads to the Quantum Motion Algebras. These algebras can be considered a good tool for analyzing quantum motion in many-body problems and also in more fundamental quantum mechanics.

Abstract: Mossbauer spectroscopy of Fe-57 and X-ray diffraction measurements in the Zr1-xTixFe2 phase have been made. Our X-ray diffraction spectra revealed that the samples have the cubic C15 crystal structure for low Ti concentration and the hexagonal C14 structure for high Ti concentration. In the range of 0.2 less than or equal to x less than or equal to 0.5 a mixture of both structures occurs. The investigated compounds are paramagnetic at RT for x > 0.80 and magnetic for x less than or equal to 0.8. In the magnetic samples we found four different values of hyperfine magnetic fields. The presence of two hyperfine magnetic fields in the C14 phase suggests magnetic moments on Fe atoms at 2a sites. The obtained epsilon (Q) vs. the x dependence indicates the spin canted antiferromagnetic structure in the hexagonal phase in the range of 0.2 less than or equal to x less than or equal to 0.8.

Abstract: Recent results of positron annihilation studies of organic solids are reviewed. The studies concentrated around two problems: the models of pick-off process and influence of external factors on positronium lifetime (or intensity) in solids. Several variants of the pick-off model appeared and the geometries other than the spherical geometry were discussed. The positron annihilation method has proved its usefulness in investigations of structural changes in the medium. However, despite of 20 years which have passed since the first attempts to obtain positronium trap properties from annihilation experiments, the situation is not fully clear; new effects are observed at extreme temperatures and under pressure.

Abstract: PAL spectra in porous Vycor glass with average pore radius approximate to 0.9 nm were measured as a function of temperature. Two long-lived components reflect the existence of two maxima in the pore radii distribution Relative intensities of these components drastically change with temperature.

Abstract: In earlier measurements of momentum distribution of annihilating e(+)e(-) pairs the intensity of o-Ps component seemed to be much higher than that expected from the Lifetime data. In this paper we show that such a deviation disappears if one assumes that the pick-off component of angular correlation of annihilation radiation is narrower by 10% than the component related to free annihilation.

Abstract: It was shown that the annihilation of free positrons produces a momentum distribution of annihilating pairs which is narrower for doped pyrene than for pure pyrene. We propose to explain this effect by enhanced annihilation on guest molecules; the enhancement factor for phenanthrene as admixture is of the order of 25. The increased annihilation probability on admixtures is not correlated with the respective effective numbers of electrons participating in annihilation per molecule (Z(eff)) observed in collisions of slow positrons with free molecules in vacuum.

Abstract: Pore formation in two Vycor glasses of different sodium and boron oxide content was investigated. At higher initial content of these oxides the pore growth and transformation appears at lower temperatures. Generally, at higher Na and B content (a softer glass) the observed o-Ps lifetime is larger.

Abstract: Two Vycor glasses of different percentages of the components were liquated between 725 K and 950 K and leached in order to produce pores. Little intermolecular voids characterized by ortho-positronium lifetime value tau (2) approximate to 1.5 ns were observed simultaneously. At the first stage of pore formation the intensity I-2 decreased considerably for both glasses, while at liquation temperatures higher than 825 K it increased again. The increase in I-2 resulted from changes in the properties of the bulk material. The growth of tau (2) in this range of temperatures was also observed.

Abstract: The authors studied deposits from combustion chambers of high- pressure engines supplied with standard fuel (SO) and ecological fuel of City-Diesel type. Chemical analysis, X-ray diffractograms, Mossbauer absorption and additionally Raman wavenumber measurements were made. The wearing of some engine elements was examined by the profilometric method. By using ecological fuel, the deposits were shown to contain four to five times less iron compounds than standard fuel supply. This accounts for a smaller attrition of the combustion chamber elements, and thus longer durability of the engine. (C) 2001 Elsevier Science Ltd. All rights reserved.

Abstract: The present paper is devoted to the synthesis and textural characterization of silica materials. The mixed silica sample was composed of Si-100 silica gel with small agglomerates of MCM41 molecular sieve deposited on its surface. MCM41 mesoporous silicates were synthesized using octadecyl trimethylammonium bromide. Using a new synthesis procedure it was possible to prepare silica containing three groups of mesopores. The recently developed positronium annihilation lifetime spectroscopy method for characterization of mesoporous materials was applied ro the investigated silica materials. The results were compared with those from the nitrogen desorption method. The current studies demonstrate moderate consistency of pore size characteristics obtained by using various techniques. (C) 2001 Academic Press.

Abstract: The model of a particle in a rectangular potential well, commonly used in positron studies of free volume distribution in polymers, was extended to larger voids, like pores. by taking into account the effects of positronium annihilation from the excited levels. The average lifetime calculated in this way (for room temperature) agrees quite well with the experimental data for various porous materials (silica gels, Vycor glass, porous polymers). The parameter DeltaR describing the penetration of positronium wavefunction into the bulk needs to be slightly enlarged in relation to that in small spherical voids.

Abstract: Modification of the Tao-Eldrup model is proposed in order to extend its usefulness to the case of porous media. The modification consists in the transition from spherical to capillary geometry and in inclusion of pick-off annihilation from the excited states of a particle in the well. Approximated equations for pick-off constant in these states are given. The model was tested by observing the temperature dependences of o- Ps lifetime in various media. In the case of silica gels and Vycor glass with narrow pores, the model seems to work well, while for larger pores in Vycor unexpectedly long lifetimes appear in the range of lowest temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.

Abstract: The formation of narrow pores during leaching of Vycor glass by sulphuric acid was investigated using the positron lifetime technique. During the leaching process the pore diameter remained roughly constant (except for the case of cold leaching). The time of processing changed the total length of capillaries, but not their number; at the temperature 50 degrees C during 20 min of leaching the average leaching depth was 24 mu m. (C) 2000 Elsevier Science Ltd. All rights reserved.

Abstract: This study concerns Vycor glass subjected to liquation in the temperature range 725-950 K and leached in hot acid in order to produce pores. Three stages of pore creation were observed with increasing liquation temperature: The initial structure of nanopores with a radius of about 0.5 nm (like in the nonliquated glass) disappears, giving way to the next group, dominant at 875 K, whose average radius is 1.15 nm, independently of the liquation time. The spread of these radii is narrow, 0.15 nm. At the highest temperatures, the intensity of this component decreases rapidly (down to 1%), and large pores begin to grow; the specific surface area diminishes. About 900 K, the total positronium formation probability decreases significantly, which can be ascribed to the partial crystallization of silica.

Abstract: In solid crystalline biphenyl, ortho-positronium (o-Ps), abundant at normal pressure, can be eliminated by application of a pressure of about 100 MPa. The lifetime of o-Ps shortens with the rise of pressure, as a result of diminishing of the free spaces in which it is trapped. However, the lifetime of para-Ps seems to increase, and at 80 MPa, it cannot be distinguished from that of free positrons. Another new effect is the reappearance of the long-lived component in the positron lifetime spectrum at high pressures. The same effect of positronium reappearance, although not so well pronounced, was found in biphenyl at low temperatures. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Abstract: The lifetime spectra of positrons in solid chrysene were measured at temperatures of 160-300 K. The temperature dependence of positronium yield suggests Ps trapping on thermally produced free volumes. Those volumes cannot be vacancies, but rather the intermolecular free spaces; thermal motions of molecules can increase these spaces to the extent making Ps creation energetically possible.

Abstract: The intensities of ortho-positronium components related to the pores and to small voids in the bulk were determined for a series of Vycor glasses. The ratio of "pore fraction" to the total o-Ps yield depends on the specific surface area of the porous medium. The diffusion coefficient for o-Ps was found equal to 3.7 x 10(-5) cm(2)/s.

Abstract: The size and shape of the free volume in molecular crystals was estimated from crystallographic data. The o-Ps lifetime values were calculated from respective models and compared with the experimental data. The best agreement for channel-like voids has been obtained for the value of DR parameter equal 0.19 nm. The threshold value of radius allowing Ps trapping in the void was found to be (0.12-0.13) nm.

Abstract: Nanocrystalline powder samples of Fe70Al30 alloy were prepared by the mechanical alloying method. X-ray diffraction, Mossbauer and magnetic studies indicated that the ferromagnetic solid solution with BCC structure was formed with an increase in milling time. The magnetic ordering temperature T-c of the nanocrystalline mechanically alloyed Fe70Al30 was larger than that of the corresponding alloy prepared by rapid quenching, while the average magnetic moment per Fe atom in the investigated milling alloy was smaller than in cold-worked Fe70Al30 alloy. The high value of the hyperfine magnetic field of about 29 T measured in the Fe70Al30 alloy was explained in the frame of a local environment model, where five or more Fe atoms in the nearest neighbourhood of Fe-57 constitute the most probable configuration. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: The ortho-positronium lifetimes in porous Vycor glass were measured in a wide range of pore radii. A good correlation of the most probable lifetime with the model accounting for the population of upper levels of a particle in the potential well was observed. For channel-like voids (capillaries in porous media) a new value of the empirical parameter, Delta R, entering in the semi-empirical Eldrup equation, is proposed: Delta R = (0.191 +/- 0.007) nm; it explains some of the discrepancies previously found in crystalline solids. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: The lifetime spectra of positrons annihilating in silica gels with pore radii from 2 to 10 nm were measured, and the results compared with the predictions made by the Tao-Eldrup model extended to include the annihilation of positronium from the excited states in the potential well. The long-lived parts of the spectra were analysed as a sum of 1-3 components with continuous lambda distributions (gaussian in the logarithmic scale). The Ip state population in Si40 is demonstrated; the decay constant of ortho-Ps in it agrees quite well with the model estimate and the growth of intensity with temperature resembles that expected from the Boltzmann distribution. In one-component fit the lifetimes at the maximum of lambda distribution at a room temperature for various pore radii were found also to be very close to the calculated lifetime averaged over all states of the positronium particle in the rectangular potential well. (C) 1998 Elsevier Science B.V. AII rights reserved.

Abstract: Ferromagnetic Laves phase compounds Zr(Fe1-xCox)(2) have been investigated by means of the Mossbauer effect (Fe-57) and by the time-dependent perturbed angular correlation of gamma-rays (Ta-181) technique. It has been concluded from ME experiments that by exchange of Fe by Co in the nearest neighbour shell of the nuclear probe the hyperfine magnetic field acting on Fe-57 decreases by 10-12 kG. The analysis of the TDPAC experiments revealed that two different hyperfine magnetic fields: B- 1(hf)(Ta) approximate to 61 kG and B-2(hf)(Ta) approximate to 88 kG act on the Ta-181 nuclei. Both have a negative sign.

Abstract: An attempt to extend the popular Tao-Eldrup model (TEM) toward large voids is presented. The lifetimes of o-Ps in various states of the particle in the spherical and cylindrical wells have been calculated. The existence of o-Ps in excited well state("1p-like" state) is demonstrated.

Abstract: The size of free volume in molecular crystals was estimated from crystallographic data. The threshold value of free volume radius allowing Ps formation was found to be (0.12 - 0.13) nm. The lifetime estimates were compared with experimental data for 13 crystalline solids.

Abstract: The lifetime spectra of positrons annihilating in a porous medium (silicagel) were measured as a function of temperature. In the low temperature limit the lifetime of o-Ps in the pores agrees rather well with the popular Tao-Eldrup formula. With the rise of temperature the second long-lived component grows; that component was identified as o-Ps annihilation from the Ip- like state in the potential well. The lifetimes of o-Ps in various states of the spherical well have been calculated. The experimental value of the lifetime for the Ip state is in good agreement with the model calculation. (C) 1997 Elsevier Science B.V.

Abstract: Lifetime spectra of positrons in solid dihydroxybenzenes have been investigated over a wide range of temperatures. The variation of Ps formation probability with temperature and relative permittivity is an argument in favour of spur model concepts in the description of Ps formation in solids. An attempt to estimate the average excess electron-positron distance in the spur is undertaken. Detailed analysis of the e(+) lifetime spectrum in hydroquinone indicates that the long- lived part of that spectrum can be a continuous distribution of lifetimes, not a discrete component.

Abstract: Temperature dependence of positronium formation probability P was measured for catechol, resorcinol and hydroquinone. The variations of P with temperature and relative permittivity can be explained in the framework of spur reaction mode. It seems that the spur processes are important at the first stage of Ps formation, the final value of P depends also on free volume availability and trapping efficiency.

Abstract: Experimental values of o-Ps lifetime were compared with model calculations, approximating the voids in a solid by spheres or cuboids. In the case of resorcinol the model of spherical voids is well consistent with the experimental data. For channel-like free volumes the formula, analogous to that for spheres, gives systematically the values of o-Ps lifetime too large. No positronium is found in anthracene, although the radius of free volume is larger than 0.97 Angstrom at which the Ps binding energy should be zero.

Abstract: Magnetic quenching experiments with naphthalene give small value of density parameter eta. Corresponding lifetime of p-Ps should be equal or close to that of free annihilation. Contrary to expectations, we obtained the lifetime of short-lived component tau(1) = 180 ps (its intensity is larger than predicted by statistical weight rule). Generally, the experiments with the use of magnetic field give smaller eta value than these using zero-field methods.

Abstract: Positron annihilation data collected for zeosil (a zeolite-type solid allowing abundant Ps formation) are discussed, with particular attention payed to the longest-lived positronium states as observed in lifetime spectroscopy experiments. To unravel the nature of these components, additional information was sought by applying an external magnetic field and by recording the total energy distribution of the annihilation radiation. From both the lifetime and the extended Doppler spectra, it is shown that positronium presents a very long- lived component with a relative intensity of ca. 30% and a lifetime close to the intrinsic tripler lifetime (140 ns), therefore undergoing almost no pick-off annihilation. From the magnetic field experiments, the contact density parameter has a value of essentially 1, which is characteristic of an unperturbed triplet Ps state in vacuo. This positronium state therefore does not appear to interact with the medium.

Abstract: The temperature dependence of parameters describing the positron lifetime spectrum in resorcinol has been investigated. It was found that the solid phase transition point is located near 353 K, but over several K the transition is very slow, its rate lies in the range of days. The dimensions of positronium traps (free volumes) were estimated from crystallographic data. In spite of the non-spherical shape of these free volumes the o-Ps lifetime and its temperature dependence calculated from the popular Tao-Eldrup formula are relatively consistent with the experimental values. The main features of the temperature dependence of the positronium formation probability can be explained in the framework of the spur model. The increase in probability in the region below the transition point may be the result of ion-molecule reactions in the spur, or of the dependence of the Ps trapping probability on the trap size.

Goworek, T.
Jasinska, B.
Wawryszczuk, J., Positrons in benzenediols - Does the spur model work in a solid?, Abstracts of Papers of the American Chemical Society, 211(), 1996, 34-NUCL

Abstract: Platinized magnesium has recently been proposed as a new reducing agent for the conversion of small quantities of water to hydrogen in a dame-sealed borosilicate glass tube at 400 degrees C for isotopic analysis. The reagent, Mg-Pt, in contrast to zinc can be prepared in every laboratory by coating a magnesium granulate with a thin layer of platinum by reaction with H2PtCl6 . 6H(2)O dissolved in acetone-ether mixture. Excellent reproducibility of the isotope ratios in hydrogen gas prepared from water samples has been obtained using 4 mu l of water and 120 mg of the reagent.

Abstract: Powder samples of Fe25Al75 Were prepared by the mechanical alloying method. Mossbauer effect, X-ray diffraction and DSC measurements indicate that Fe and Al crystalline powder transform into Fe-Al amorphous powder with increasing milling time. The X-ray diffraction patterns of the milled Fe25Al75 do not clearly show a sign of the existence of the intermetallic phases or Fe-Al solid solution. However, Mossbauer measurements reveal two sites with hyperfine magnetic fields 30.2 and 26.0 T. These sites form locally during the milling process and then they disappear.